1. Field of the Invention
The present invention relates to image processing for generating image data for use in image formation by multi-pass recording.
2. Description of the Related Art
In multi-pass recording, a recording medium is conveyed in the interval between successive recording scanning operations, thus ink droplets are supplied onto the recording medium at a predetermined time interval. Hence, an image can be recorded even on a recording medium such as plain paper which absorbs ink at a relatively slow rate while gradually drying the supplied ink droplets, thereby obtaining a satisfactorily result upon fixing. Also, in the multi-pass recording, different nozzles are used to record the same image region for each recording scanning operation upon conveying the recording medium. Hence, even if the individual nozzles have a variation in ink discharge amount between them, the variation in discharge amount can be canceled and made inconspicuous on the image. Further, heterogeneity of density (so-called white streaks and black streaks) is often generated due to a variation in amount of conveyance of the recording medium in the interval between successive recording scanning operations, but can be made inconspicuous by the multi-pass recording.
Note that a variation in discharge amount between the individual nozzles and that in amount of conveyance of the recording medium lead to image deterioration, which are unavoidable due to factors associated with the manufacturing process and component accuracy. Therefore, the multi-pass recording is an important technique in maintaining a given image quality in a serial inkjet recording apparatus.
The recording rate in each pass of the multi-pass recording by an inkjet recording apparatus is given by 1/(the number of passes). That is, in four-pass recording, each pass has the same recording rate of 25%. However, the invention disclosed in Japanese Patent Laid-Open No. 2002-096455 changes the recording rate in each pass in accordance with the positions of recording elements (nozzles). This reduces image deterioration such as so-called “color heterogeneity” generated due to the difference in the applying order of different inks, and heterogeneity of density due to so-called “end dot deflection” in which the ink-landing positions of liquid droplets discharged by nozzles in the end portions of a nozzle array shift more significantly than those of liquid droplets discharged by nozzles in its middle portion.
FIG. 1 illustrates an example of the relationship between nozzles and the recording rate in the four-pass recording. Referring to FIG. 1, the axis of abscissas indicates the nozzle number (the numbers 0, 1, 2, . . . assigned to nozzles in turn from the end of a nozzle array in the sub-scanning direction), and the axis of ordinates indicates the recording rate. The recording rates in the end portions of the nozzle array are less than 25%, that in the middle portion of the nozzle array is more than 25%, and the average recording rate is 25%, as shown in FIG. 1. That is, the recording rate is higher in the middle portion of the nozzle array than in its end portions, so the end dot deflection is reduced and image deterioration, in turn, is reduced.
On the other hand, dye inks formed using dyes which easily dissolve in water as color materials are widely employed as inks for an ink jet recording apparatus. In a dye ink containing water as its major component, the color material dissolved in a solvent easily penetrates into the fibers of a recording medium. Hence, even after image recording, the surface shape of the recording medium is easily maintained, so a gloss of the recording medium itself is maintained intact as that of an image. In other words, an image with an excellent gloss can be easily obtained upon recording an image on a recording medium with an excellent gloss using dye inks. Hence, an inkjet recording apparatus which employs dye inks can adjust the glossiness of an image by adjusting the glossiness of a recording medium.
A dye ink generally has low lightfastness, so dye molecules of the color material photo-decompose and the formed image fades. Also, a printing product printed by a dye ink generally has low water resistance, so dye molecules penetrated into fibrous materials dissolve in water as it gets wet, and a smear is generated in the formed image.
To solve problems associated with the lightfastness and water resistance, which are encountered when dye inks are used, development of pigment inks formed using pigments as color materials is in progress in recent years. A pigment ink contains particles of a pigment with sizes of several tens of nanometers to several micrometers in a solvent, unlike a dye ink which contains molecules of a dye. Color material particles of a pigment ink are larger than those of a dye ink, so a printing product with high lightfastness and water resistance can be obtained using the former ink.
The color material of a pigment ink is hard to penetrate into a recording medium, and therefore deposits on the surface of the recording medium. Thus, the microscopic shape of the image surface differs between a region to which a pigment ink is applied and that to which no pigment ink is applied. Also, the amount of color material used differs depending on the image density and color. Accordingly, the area across which the color material covers the recording medium differs in that case, and the reflectance of the color material and the surface reflectance of the recording medium are different from each other, so a difference occurs in glossiness depending on the difference in area across which the color material covers the recording medium.
For the above-mentioned reason, when an image is recorded using a pigment ink, the sense of glossiness differs depending on the image density and color. Also, even if the image density and color are the same, regions with different glossinesses appear in a band shape with a width corresponding to the conveyance distance of a recording medium, as will be described in more detail later. The state in which regions with different glossinesses are mixed in the same image, as described above, will be referred to as “heterogeneity of glossiness”, and heterogeneity of glossiness appearing in a band shape will be referred to as “band-shaped heterogeneity of glossiness” hereinafter. When this occurs, a gloss region in which a gloss is observed and a matte region in which no gloss is observed are mixed in the same image, and one recognizes this image as an unsatisfactory image especially when it is a photographic image.
To suppress the heterogeneity of glossiness, a method of using ink (to be referred to as clear ink hereinafter) that is substantially transparent and colorless and therefore does not influence color reproduction has been known. That is, the heterogeneity of glossiness is suppressed by applying clear ink or white ink to a region which is covered with no color ink (for example, Japanese Patent Laid-Open No. 2002-307755). The inventors of the present invention conducted a close examination, and found out that the technique disclosed in Japanese Patent Laid-Open No. 2002-307755 is effective for heterogeneity of glossiness generated due to the difference in density or color, but is ineffective in reducing band-shaped heterogeneity of glossiness generated even when the image density and color are the same.
When multi-pass recording is performed at a recording rate which differs for each recording scanning operation, band-shaped heterogeneity of glossiness appears for each conveyance width (each conveyance distance in the sub-scanning direction) of recording paper per recording scanning operation. The band-shaped heterogeneity of glossiness will be described with reference to FIG. 2. A recording region on a recording medium is divided for each conveyance width, and the obtained recording regions are defined as a first recording region, second recording region, and third recording region in turn in the sub-scanning direction, as shown in FIG. 2. The glossiness changes from the upper end to the lower end in each recording region, and a large difference in glossiness occurs between the ends of adjacent recording regions (for example, the lower end of the first recording region and the upper end of the second recording region) and is recognized as band-shaped heterogeneity of glossiness.
The cause of the difference in glossiness between the upper and lower ends of each recording region will be explained with reference to schematic views shown in FIGS. 3A and 3B. When two liquid droplets to be superimposed on each other upon landing are discharged in the same pass, the second liquid droplet lands and is superimposed on the first liquid droplet, before the first liquid droplet sufficiently dries, so these two liquid droplets merge into one liquid droplet (FIG. 3A). However, when two liquid droplets to be superimposed on each other upon landing are discharged in different passes, the second liquid droplet lands after the first liquid droplet dries, so these two liquid droplets do not merge into one liquid droplet (FIG. 3B). As a result, the surface of a dot formed by liquid droplets superimposed on each other in the same pass becomes smooth (has high glossiness), while that of a dot formed by liquid droplets superimposed on each other in different passes becomes rough (has low glossiness).
The states of the surfaces of the upper and lower ends of a recording region in four-pass recording will be described with reference to schematic views shown in FIGS. 4A and 4B. Note that the nozzle recording rate in each pass is the same as that shown in FIG. 1.
The recording rate at the upper end is 26% in the first pass, 32% in the second pass, 26% in the third pass, and 16% in the fourth pass, and this means that the amount of ink recording (58%) in the first half pass is larger than that (42%) in the second half pass. On the other hand, the recording rate at the lower end is 16% in the first pass, 26% in the second pass, 32% in the third pass, and 26% in the fourth pass, and this means that the amount of ink recording (42%) in the first half pass is smaller than that (58%) in the second half pass. At the lower end at which the amount of ink recording in the second half pass is relatively large, dots with a relatively small amount of ink recording are covered with those with a relatively large amount of ink recording, so the surface has its uneven shape lessened and has high glossiness (FIG. 4B). Conversely, at the upper end at which the amount of ink recording in the second half pass is relatively small, dots having a relatively small amount of ink recording are formed (dots are sparsely formed) on those which are formed first and have a relatively large amount of ink recording, so the surface has its uneven shape enhanced and has low glossiness (FIG. 4A). In this manner, a difference in glossiness occurs between the upper and lower ends of the recording region, thus generating band-shaped heterogeneity of glossiness.